The mechanism of the phosphorylation-dependent myosin-linked regulation of smooth muscle myosin is being investigated. This involves several approaches: (1) to identify the rate-limiting step in the acto-myosin MgATPase cycle for both phosphorylated and unphosphorylated heavy meromyosin (HMM), the proteolytic subfragment of myosin; (2) to determine which step(s) is affected by phosphorylation and by how much; (3) to use an in vitro motility system to quantitate how the velocity of movement of myosin-coated beads is affected by various factors, such as calcium concentration, magnesium concentration, and extent of phosphorylation. The mechanism of the phosphorylation-dependent regulation of non-muscle or cytoplasmic myosin is also being investigated.